This invention relates to disk drives in general and more particularly to a method and apparatus for accurately restoring the DC signal level of recorded servo information which has been electronically recovered from a rotating disk storage device.
A block diagram of a typical servo recovery channel for a disk drive is shown in FIG. 1. The recorded servo information on disk 11, which has been recovered by the servo head 13, is then amplified and filtered by preamplifier 15, low pass filter 17, amplifier 19, and band pas filter 21 to amplify the signal while eliminating the unwanted frequency components. An automatic gain control (AGC) loop 23, including variable gain amplifier 25, amplifier 27, and the AGC sense and feedback circuit 29 then amplifies the servo signal to its final value determined by the reference voltage Vagc, and maintains it at that value through the action of the feedback loop.
Since amplifiers 25 and 27 are commonly DC-coupled, a significant shift in the DC component of the signal occurs. This is corrected by the automatic DC restore circuit 31, which accurately restores the DC level (baseline) of the signal to a value (determined by Vref) which is maintained constant regardless of amplifier bias conditions or signal degradation. The analog servo data signal is then demodulated with reference to its DC-restored level by peak detectors 33, and is also converted to digital timing pulses by pulse digitizer 35. So that accurate head position and disk timing information can be obtained, it is essential that the servo signal be DC-restored with respect to its true signal baseline so that demodulation and digitization can reliably occur.
Two methods are commonly used to perform DC restoration in the field of disk storage devices. The first and most obvious solution is to passively AC couple the output of the servo data amplifier 27 to any subsequent circuits. Since the AC coupling capacitor effectively eliminates the low impedance drive capability of the amplifier, it then becomes necessary to restore this feature with a high performance buffer.
A more common approach used in the prior art, however, is depicted in FIG. 2. In this method, the DC restore circuit 31 includes a low pass filter 41 and integrating amplifier 43. The signal present at the output of amplifier 27 is filtered by low pas filter 41 to attenuate the AC components of the signal, while allowing the DC and low frequency components to pass. An integrating amplifier 43 then compares this DC level with a reference voltage (typically ground), and generates an error voltage which is then fed back to the input of amplifier 27. Due to the application of negative feedback, the bias conditions of amplifier 27 will be adjusted to correct for any DC offset present at Vout.
Since it is the object of the aforementioned method to extract the DC components of the signal with the use of a low pass filter, there is in practice a major disadvantage which attends such designs. Due to the action of the low pass filter, a pseudo-baseline (DC) level is created by time-averaging the positive and negative excursions of the signal. This DC level will be affected by any distortion present in the servo signal, which in practice is common due to an imperfect head-disk interface (HDI) or the poor performance of the preceding servo recovery circuits, thus exhibiting an offset with respect to the true signal baseline. In actuality, this offset is an error inherent to the operation of this type of DC restore circuit which will be passed on to any subsequent circuits.
In addition, since it is the object of the prior art method to adjust the DC level at the output of amplifier 27 by adjusting the input bias conditions of the amplifier, there are additional disadvantages which must be considered. For instance, since bias correction is often applied to the input stage of the amplifier, the input impedance of the amplifier will vary thus affecting the performance of the preceding circuits. A reduction in the dynamic range of the amplifier will also be observed since the DC restore circuit must compensate for component tolerances inherent in the amplifier itself.
It is therefore an object of this invention to provide a novel DC restore apparatus which eliminates the aforementioned disadvantages common to prior art methods of DC restoration. Related patent son this subject are U.S. Pat. Nos. 4,373,140; 3,846,710; 3,579,123; and 3,557,305.